In optical particle detectors, the presence of particles is detected in an air sample by monitoring the extent of scattering from a beam of electromagnetic (EM) radiation traversing the sample. From time to time undesired impurities, such as dust, lint or insects may enter the detection chamber in which detection takes place. If such impurities impinge upon the beam of EM radiation they will typically cause a great deal of scattering within the detection chamber and may cause a “false positive” detection event. This is particularly the case if the unwanted impurities enter the portion of the beam that is visible to the EM detector which is typically called the “region of interest”.
In order to guard against the entry of undesired impurities into the detection chamber many particle detectors are provided with filters in the flow path to filter the sample prior to it entering the detection chamber.
One method of filtering the inlet of a smoke detection system, which is used primarily in aspirating smoke detectors is filtration using “bulk” filters, such as foam, paper or the like. Such filters are effective at removing both large particle dust particles and elongate particles like lint, but suffer from a tendency to also remove the smoke particles that are intended to be detected.
The removal of the wanted particles (i.e. smoke) from the air-flow by bulk filters becomes increasingly severe as the filter material becomes clogged. However, this clogging has been discovered to have proportionally less severe effect on the air-flow through the system, than it does on the removal of smoke particles passing from the air-flow. The consequential effect on pressure drop across the filter is therefore relatively small. Moreover, such filters are generally designed to contribute only a small fraction of the pressure drop of the whole system meaning that the effect of its clogging upon flow rate cannot be relied upon to signal an unacceptable loss of sensitivity.
An alternative to the use of a bulk filter is to use a mesh screen that is placed across the inlet to the detector. Such screens are effective at preventing particles with all dimensions greater than the hole size of the mesh screen from entering the detection chamber. However, smaller particles still get through. Elongate particles also pose a problem for mesh screen filters, because, although they may be much longer than hole size of the mesh screen, the particles may have sufficiently small cross section to pass through the screen if they are aligned with it.
Moreover, some elongate particles, which are initially stopped by a mesh screen, will work their way through the screen by changing orientation with respect to the screen to thereby “wiggle” their way through it. In the past it has been identified that finding a mechanical solution to this problem was not possible and that a software-based solution was thought to be required.
A range of techniques enable a smoke detector to tolerate the effects of dust particles without the use of a filter are known, however elongate particles can still pose a problem as they may become lodged within the detection area of the smoke detector and trigger a substantially permanent false smoke detection response.
In practice some elongate particles will move through the region of interest relatively quickly, and cause transient false positive readings. Typically such events can be handled using suitable algorithms in the detector software. However, a permanent false alarm may be caused by particles that become lodged in, or adhere to, the inside of the detector in a position that causes them to remain in the region of interest. The length of particle that can induce such a false alarm is dependent upon the internal architecture of the particular smoke detector. In this, regard, only particles that are long enough to extend from a surface, on which it may adhere, to the region of interest will pose a threat to the detector. Shorter elongate particles will not extend into the region of interest and thus do not pose a threat in this manner.
It is to be understood that the discussion of prior art herein does not constitute an admission that that prior art, or the conclusions drawn from it by the inventor, form part of the common general knowledge of a person skilled in the art at the priority date of the application.